ε-Caprolactone is heated with an initiator having an active hydrogen such as a hydroxyl group in the presence of a catalyst to allow ring-opening polymerization. Although aliphatic polyesters are generally considered to have difficulty in having a higher molecular weight, ε-caprolactone is polymerized to easily prepare an aliphatic polyester having a high molecular weight. Therefore, ε-caprolactone is used as materials for many polyesters. However, the reaction rate depends on the concentration of ε-caprolactone and depolymerization causing ε-caprolactone regeneration occurs at a high temperature, so that it has a limitation to decrease the concentration of ε-caprolactone by a reaction. In particular, with respect to a copolymer with a polymer having a high melting point, the reaction temperature should be maintained at a high temperature, so that the influence of depolymerization cannot be ignored.
For a polymer obtained by a reaction of a polymer having a hydroxyl group and/or an ester bond and a cyclic ester containing ε-caprolactone, processes for producing polyester block copolymers each obtained by a ring-opening polymerization of a crystalline aromatic polyester and a lactone are described in patent publications such as JP 48-4116 B, JP 52-49037 B, JP 8-9661 B, JP 7-76263 B, and JP 3-263425 A.
Of those, the publications of JP 52-49037 B, JP 8-9661 B, JP 7-76263 B, and JP3-263425 A disclose processes for removing monomers at temperatures equal to or higher than melting points under reduced pressure as processes for removing remaining monomers. Although all of the processes include treating a melt under reduced pressure, those publications show no specific process for recovering ε-caprolactone. Actually, those processes are insufficient when being performed continuously for a long period of time.
On the other hand, each publication of JP 60-31525 A or JP 5-23290 B describes a process for producing a polymer in which a polyester-based block copolymer obtained by a ring-opening polymerization of a crystalline aromatic polyester and a lactone is further polymerized in a solid-phase state at a temperature of 170 to 215° C. and below the melting point so as to have a higher molecular weight. Although both processes include heating treatments under vacuum or in a gas stream, those publications describe no process for recovering ε-caprolactone to be removed.
An object of the present invention is to provide a process for recovering ε-caprolactone, which is contained in an ε-caprolactone polymer and causes a trouble in industrial production of an ε-caprolactone polymer, to a process for producing an ε-caprolactone so as to enable continuous operations.
To attain the aforementioned object, the inventors of the present invention have made extensive studies. As a result, the inventors have found out that ε-caprolactone remaining in a polymer can decrease by specifying the weight ratio of ε-caprolactone to be recovered relative to caprolactone dimer, and the present invention has been accomplished.